The basic operation and structure of communication systems, such as cellular radio telephone systems communication systems and land mobile communication systems, are well known in the art. Communication systems typically comprise a plurality of communication units, a predetermined number of base station (or repeaters) located throughout a geographic region and a controller. The communication units may be vehicle mounted or portable units and comprise either a transmitter or a receiver or both to form a transceiver. The communication unit is coupled to the base station by a communication channel over which radio frequency (RF) signals are transmitted and/or received. The controller comprises a centralized call processing unit or a network of distributed controllers working together to establish communication paths for the communication units.
It is well known in the art for transmitters to include an automatic signal power level control loop to maintain the output power level of a transmitted RF signal at one of a plurality of predetermined power levels. Automatic signal power level control loops generally typically comprise an RF coupler, a power level detector and a processor. The RF coupler couples a portion of the transmitted RF signal to the power level detector. The power level detector detects the power level of the transmitted RF signal to produce an output signal. The processor adjusts the power level of the transmitted RF signal responsive to the output signal to maintain the output power level of the transmitted RF signal at one of the plurality of predetermined power levels.
FIG. 1 illustrates a schematic diagram of a first prior art RF coupler 100. The RF coupler 100 generally comprises a primary transmission element 101 and a first secondary coupling element 102. The primary transmission element 101 receives at its input an RF input and produces an RF output at its output. The first secondary coupling element 102 couples a portion of the RF signal to produce a coupled RF output signal. The physical dimensions of the primary transmission element and the first secondary coupling element and the distance 103 from the primary transmission element determine the amount of the RF signal coupled.
FIG. 2 illustrates a schematic diagram of a second prior art RF coupler 200 including the primary transmission element 101 and the first secondary coupling element 102 as of FIG. 1. The first secondary coupling element 102 couples at least a portion of the RF signal to produce a first coupled RF output signal. A second secondary coupling element 201 couples at least a portion of the RF signal to produce a second coupled RF output signal. The first and second coupled RF signals may be adjusted by varying the dimensions of the secondary coupling elements 102 and 201 and by varying the spacing 103 and 202, respectively, between each secondary coupling element 102 and 201 and the primary transmission element 101. However, a disadvantage of this structure is cross coupling between the first and the second secondary coupling elements 102 and 201.
FIG. 3 illustrates a schematic diagram of a third prior art RF coupler 300 overcoming the disadvantage of the RF coupler of FIG. 2. Instead of placing the second secondary coupling element 201 opposite to the first secondary coupling element 102, the second secondary coupling element 201 is offset from the first secondary coupling element 102 to substantially reduce the cross coupling between the secondary coupling elements 102 and 201.
The range of coupled transmitted RF signal power levels at the power detector's input for which the power detector's output signal is usable is commonly referred to as the power detector's dynamic range. Some communication systems now require the transmitters to operate over a wider range of signal power levels to communicate with the base stations. This requirement can be met by increasing the dynamic range of the power detector.
The dynamic range of the power detector is related to the power level of the coupled transmitted RF signal. At high transmitter power levels, RF couplers couple a large mount of signal power level for detect/on by the power detector. However, a significant portion of the coupled transmitted RF signal is lost at high power levels. Therefore, the current drain of the transmitter is increased to overcome the loss of power at high power levels which reduces the transmitter's efficiency. At low transmitter power levels, RF couplers provide a low amount of signal power level for detection by the power detector. However, at the low transmitter power levels there may not be enough signal power available at the power detector's input to generate a usable output signal.
Therefore, there is a need for an RF coupler providing variable RF coupling of the transmitted RF signal to increase the dynamic range of the power detector in the transmitter such that the transmitter can operate over a wider range of predetermined power levels.